Design a large-scale Payment Platform
Last updated: January 15, 2026
Quick Overview
Design a scalable payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Brex
System Design
Software Engineer
Brex
January 15, 2026Software Engineer
Technical Screen
System Design
Hard
0
5
2,430 solved
Design a scalable payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This ML system design question from Brex's Technical Screen tests your ability to think about ML systems at scale. The interviewer expects discussion of data quality, feature stores, model serving infrastructure, and A/B testing strategy.
What the Interviewer Expects
- Design the full ML lifecycle from data collection to model monitoring
- Address cold start, exploration/exploitation, and model freshness
- Discuss multi-objective optimization and ranking systems
- Plan for model debugging, fairness, and bias mitigation
- Design the feature store and training pipeline for scale
- Address model versioning, canary deployments, and rollback strategies
- Discuss the data flywheel and long-term system evolution
Key Topics to Cover
Training pipeline and infrastructure
Model serving and latency optimization
A/B testing and experimentation
ML objective formulation and metric selection
How to Approach This
- Start by clarifying functional and non-functional requirements with the interviewer.
- Estimate the scale: QPS, storage, bandwidth. This drives your design decisions.
- Draw a high-level architecture first, then deep dive into 1-2 critical components.
- Discuss trade-offs explicitly (e.g., consistency vs availability, SQL vs NoSQL).
- Address failure scenarios, monitoring, and how the system handles 10x traffic spikes.
Possible Follow-up Questions
- How would you run A/B tests on different model versions?
- What is your model retraining strategy?
- What would you do if model performance degrades over time?
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Requirements
Functional Requirements
- Payment Processing: The system must process various payment methods (credit cards, ACH transfers, etc.) efficiently.
- Transaction History: Users should have ac...
Capacity Estimation
Assuming Brex processes around 1 million transactions per day:
- Requests per Second (RPS): 1,000,000 transactions / 86400 seconds = ~11.6 RPS.
- Peak Load: Assume peak hours process 10x the a...
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